Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211142

Area Forecast Discussion
National Weather Service Sioux Falls SD
542 AM CST Tue Feb 21 2017

.SHORT TERM...(Today and Tonight)
Issued at 413 AM CST Tue Feb 21 2017

Weak ridge axis settling southward across northwest Iowa early
this morning, with moisture from yesterday`s rainfall and ideal
radiative cooling enough to produce some patchy shallow fog
near/east of I-29 favoring mainly lower elevation and southerly
upslope locations of the Buffalo ridge. Gradient does continue to
increase gradually through the predawn hours, which should start
to minimize patchy fog coverage and favor gradually more southward
into northwest Iowa. This is similar to treatment by RAP/HRRR
solutions through early morning.

Beyond the fog early morning, today is shaping up to feature
spectacular warmth.  Record temperatures are a certainty for many
locations with very minimal negative impacts on warming toward upper
limits today, with a strong westerly component, minimal cloudiness,
and a drier airmass. Highs looking to reach the lower to mid 70s
through the Missouri River corridor, gradually decreasing toward mid
60s across southwest Minnesota.

Wave riding flattening ridge tonight will glance the far northern
portions of the forecast area. A little mid-level frontal support
for cloud development the latter half of the night heading north
from I-90, but with stout dry layer below, do not expect even any
sprinkles to fall from this cloud layer.  Should again see a fairly
distinct elevation signature to temperatures overnight while in warm
sector, as surface gradient relaxes and decouples for lower
elevations allows for mid to upper 30s, while slightly stronger
winds and higher position in the inversion keeps lower to mid 40s in
play along the Buffalo Ridge and toward south central SD.

.LONG TERM...(Wednesday through Monday)
Issued at 413 AM CST Tue Feb 21 2017

The big story in the medium range forecast continues to be the
abrupt change from record highs to a strong winter storm at the end
of the week.

Wednesday: Shortwave energy zipping through the Northern Plains will
bring an enhancement to mid-level clouds, however soundings continue
to indicate a very dry subcloud layer.  Have kept sprinkles out of
the forecast through the morning.  A frontal boundary passes through
the area early in the day, with cold advection aloft through the
afternoon. Temperatures should still rise into the upper 50s to mid
60s along the highway 20 corridor.

Thursday-Friday: While model spread continues, overnight models have
again trended in different directions, but closer to a bit of a
consensus. The latest ECMWF has lifted slightly further north of
it`s previous run, the GFS has again lifted back northward falling
in line and nearly along the mean of the GEFS ensembles. The GEM
remains an outlier, both with the open nature of the initial PV
anomaly, and the lag in timing with the secondary wave on Friday
that will intensify the trough in general.   A closer look at the
GEFS ensembles allows a bit of a QPF base to be established, as very
view members now show the minimal QPF indications of earlier. While
there is still considerable spread in the solutions, there are
several clusters that focus around the deterministic solutions
narrowing in the 25 and 75 percentiles.   The GEFS probabilistic
guidance has also narrowed considerably over the past 24 hours, with
rising and narrowing threshold probabilities indicative of that
growing consensus. So have based the forecast on a blend of the GEFS
mean, EC and to a lesser degree GFS/GEM.  Have generally discounted
the 00Z NAM given it`s lower skills at extended portions of it`s run.

The real keys to this forecast revolve around depth of an east-west
oriented trough over the Canadian provinces, and stage setter wave
on Wednesday that will help determine northward extent or
development of the low level baroclinic zone on Thursday. The trend
have been to flatten the Canadian trough, allowing Wednesday`s wave
slide further north over North Dakota/Minnesota.

Getting down to the details of the forecast, still strong
indications that 700:600 mb frontogenesis will be the initial
forcing mechanism that drives precipitation development through the
day on Thursday.  Of course, we`ll be fighting dry air intrusion
from the northeast and east at the low levels, and warming surface
temperatures.  However, all models show that once more intense
precipitation begins to fall, surface temperatures cool to support
more snow. Areas east of I-29 could be more problematic due to the
advection of warmer surface temperatures westward and changeover to
snow may linger until the evening.

Initial frontogenesis will be reinforced by strong synoptic forcing as
a PV anomaly enters the Plains in the evening. As this wave begins
to deepen, forcing should shifts toward the 900:800 MB layer. EC/GFS
continue to push 850mb low track slightly further north overnight
now along a Lincoln to Des Moines to La Crosse into mid-day Friday.
This would keep the large deformation band stalled across and south
of a line from Yankton to Windom.

Winds will complicate matters with this storm, and as the surface
low shifts ever so slightly northward, so will the strongest
gradient winds. Have bumped up sustained winds to 25 to 30 mph
with gusts potential up to 40-45 mph based on low level flow on
the west side of this system. Have placed blowing snow in grids
once snow is on the ground. The initial snow accumulation
projections continue to focus the highest risk of 6" inches south
of Interstate 90, with the greatest risk of 8 or more inches in
parts of Northwest Iowa and adjacent areas of Southern Minnesota
trailing westward along the Missouri River. Further north, a tight
snow gradient may once again form due to the northerly fetch of
dry air. Will not be issuing a winter storm watch at this time,
but one may likely be needed later today.

Next Weekend: High pressure slides in behind this system, and have
lowered daily highs to account for new snow cover. There`s still
some potential for light precipitation Monday into Tuesday, but this
continues to be a minor impact.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 542 AM CST Tue Feb 21 2017

Very spotty fog will continue through around 14z across parts of
southwest MN and northwest IA. Otherwise, expect VFR conditions to
persist through 12z Wed. Increasing winds off- surface will also
work to create a small window for low- level wind shear for KFSD
and KSUX early this morning before mixing begins to bring stronger
winds to the surface.


Issued at 413 AM CST Tue Feb 21 2017

A drying of the airmass today will occur with a better westerly
component to winds. Diurnal mixing will allow humidity levels to
drop to 20 to 25 percent by early to mid afternoon through the
Missouri River corridor, with increasing westerly winds at a gusty
15 to 30 mph. Conditions should stay shy of critical with only an
hour or two of lowest humidity, and winds remaining generally
below 25 mph, but enough to reach a very high rangeland fire
danger through the Missouri River Corridor eastward through
northwest Iowa. While it will be cooler on Wednesday, there will
still be potential for a very high fire danger toward Lower Brule
locations with humidity 25 to 35 percent and northwest winds 15 to
25 mph.




SHORT TERM...Chapman
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